Gas/liquid contacting apparatus

ABSTRACT

Gas/liquid contacting apparatus comprising a vertical column with a plurality of substantially horizontal trays arranged one above the other in the column, each tray having a plurality of apertures for ascending gas and at least one discharge device for descending liquid, the discharge devices being open at their upper ends and extending at least partly below the relevant trays, the parts of the liquid discharge devices extending below the trays being provided with liquid discharge openings, wherein the liquid discharge devices are each provided with a gas passage having at least one gas entry arranged in the lower part of the discharge device and having a gas outlet arranged substantially above the upper end of the discharge device.

This is a continuation of application Ser. No. 711,873 filed Mar. 14,1985, now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to a gas/liquid contacting apparatus comprising acolumn having its interior provided with perforated gas/liquidcontacting trays for effecting intimate contact between liquid and gasflowing counter-currently through said column.

More in particular, the present invention relates to a gas/liquidcontacting apparatus comprising a normally vertically extending columnhaving a plurality of substantially horizontal trays arranged one abovethe other in the column, each tray having a plurality of apertures forthe passage of ascending gas and at least one discharge device fordescending liquid, the discharge devices being open at their upper endsand extending at least partly below the relevant trays, the parts of thedischarge devices extending below the relevant trays being provided withliquid discharge openings. In operation, liquid is introduced at the topof the column and the gas phase at the bottom of the column, so that thedifference in density constitutes a driving force causing the liquid andthe gas to flow through the column. The trays define a series ofcontacting zones wherein the liquid and the gas are brought together inintimate admixture for contacting.

The term "gas" as used throughout this specification and the claims isto be understood to include vapor.

Apparatuses of the above-mentioned type may be suitably used forcarrying out processes like distillation, absorption and stripping.Examples of trays which may be applied in such an apparatus are gridtrays, sieve plate trays which are formed of plates that are punched orotherwise fabricated to have circular or other apertures for gaspassage, and valve trays which are sieve trays provided with valveswhich are displaceably arranged in the tray-apertures. Liquid istransferred from one tray level to another via liquid discharge means,hereinafter called downcomers, mounted in openings of the trays. Theliquid discharge openings in the lower parts of the downcomers are soarranged that these openings are properly sealed against upward flow ofgas during operation of the column. The downcomers normally extendpartly above the relevant trays to establish a liquid level on the traysin the contacting zones.

If the contacting trays are to accomplish their mass transfer functionefficiently they should be supplied with de-aerated liquid from the trayabove in order to minimize vapor back-mixing between the contactingzones. The downcomers in which the liquid from the contacting zones iscollected should allow the separation of the liquid and gas phaseswithin them.

The maximum capacity of a column provided with contacting trays havingliquid downcomers is reached when either the dispersion of liquid andgas fills the whole space between adjacent trays, resulting in floodingof the column, or when the pressure drop over the trays can no longer bebalanced by the liquid holdup in the downcomers, also resulting inflooding of the column.

Within the oil and gas industries there are a number of operationsinvolving counter-current gas/liquid contacting under conditions of highgas density and low interfacial tension between the liquid phase and thegas phase, for example, fractionation near the critical region. Thesehave traditionally been a source of concern with regard to both capacityand operational stability. Over a wide range of conditions it has beenfound that for a given liquid flow rate, the maximum possible gasloading through a given tray is determined from a constant maximum loadfactor. High pressure operation, however, may cause a sharp decrease ofthe load factor at maximum capacity, due to a decrease of the densitydifferences between the fluid phases.

The object of the invention is to provide a gas/liquid contactingapparatus which is suitable for high pressure operation in that theadverse influence of high pressure on the maximum capacity of theapparatus is eliminated or at least minimized.

SUMMARY OF THE INVENTION

The gas/liquid contacting apparatus according to the invention theretocomprises a normally vertically extending column having a plurality ofsubstantially horizontal trays arranged one above the other in thecolumn, each tray having a plurality of apertures for the passage ofascending gas and at least one discharge device for descending liquid,the discharge devices being open at their upper ends and extending atleast partly below the relevant trays, the parts of the liquid dischargedevices extending below the trays being provided with liquid dischargeopenings, the apparatus being characterized in that the liquid dischargedevices are each provided with a gas passage having at least one gasentry arranged in the lower part of the discharge device and having agas outlet arranged substantially above the upper end of the dischargedevice.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a vertical section of an intermediate part of a trayedcolumn according to the invention.

FIG. 2 shows cross section II--II of FIG. 1.

FIG. 3 shows a perspective view of a downcomer provided with gasdischarge means according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

During operation of the above proposed apparatus gas collected in theliquid discharge devices also called downcomers, is allowed to escapevia the gas passages to the gas zones above the trays.

The principle of venting entrained gas from the downcomers in trayedcolumns is well known as such and is described in, for example, U.S.Pat. No. 2,247,485. The known venting systems, however, are onlysuitable for moderate pressure operations and fail to providesubstantial benefits at high pressure operations where the densitydifferences between liquid and gas have become small. In the knownventing systems entrained gas is discharged from the downcomers at alevel directly adjacent the bottom of the weirs of the downcomers. Theweirs are to be understood as those parts of downcomer sidewallsextending above the trays. Such vent systems relieve the pressure whichcan build up between the liquid flow into the downcomers and the weirsat very high liquid loads. This pressure built up is normally denotedwith the expression "pressure under the nappe of the weir". Such a ventsystem for relieving the pressure under the nappe is effective forincreasing the maximum liquid load at low and moderate pressureoperations. At low and moderate pressures, liquid falls over the weirsof the downcomers under gravity while substantially not being hamperedby the gas. The liquid throw over the weir increases with increasingliquid flow rate, and eventually reaches an opposite wall, which may bethe column wall or the opposite side of the downcomer depending on thelocation of the downcomer. At low liquid rates there is ample downcomercross section for removal of disengaged gas. As the liquid rate isincreased the space available for upwardly flowing disengaged gas isreduced to pockets of cross section which become available in a randommanner. Eventually, the entry side of the downcomer is completelyclosed, and a buildup of pressure due to disengaged gas between theliquid flow and the downcomer sidewall takes place. This pressure underthe nappe forces the liquid flow upward and has the effect of increasingthe required head on the weir. By venting the gas causing the pressureunder the nappe a lower liquid level may be maintained over the weirthereby effectively increasing the liquid capacity compared to anunvented system.

At high pressures the behavior in the downcomers is, however, quitedifferent from the one described above. At low and moderate pressuresthe drag forces on the liquid droplets entering into the downcomers isrelatively low and the liquid is substantially not hampered in theirdownward flow by the stationary gas in the downcomers. As, however, thegas density increases and the surface tension between liquid and gasdecreases, the drag forces become more important. As an absolute limit,the maximum throughput of a column may well be determined by the pointat which the gas velocity becomes equal to the maximum liquid terminalfalling velocity. This is the reason why the capacity of liquid/liquidcolumns is often correlated with the terminal velocity of the dispersedliquid phase. If the drag forces are sufficiently high, the liquiddroplets very quickly reach their terminal falling velocity. The flow inthe upper, liquid-dispersed, regions of the downcomers may beapproximately described by the following equation: ##EQU1## U_(g)=superficial downcomer vapor velocity V_(d)∞ =liquid droplet terminalfall velocity

α_(d) =dispersed (liquid) phase volume fraction

Setting U_(g) =0 for no vapor carry under, the following simplifiedequation can be written: U_(dc) =α_(d) (1-α_(d))V_(d)∞. For constantV_(d)∞, this equation predicts that U_(dc) is maximized if α_(d) isequal to 0.5. If α_(d) =0.5 then U_(dc) =0.25 V_(d)∞ for no vaporcarry-under. From this equation it seems likely that substantial vaporcarry-under will occur if U_(dc) /V_(d)∞ is greater than 0.25.Experiments have been carried out in which the liquid fraction wasmeasured at the bottom of a downcomer for a wide range of high pressuresystems. From these experiments it could be seen that the expressionabove indicates the onset of substantial vapor carry-under rather well.

It has now been discovered by Applicant that the maximum downcomerliquid load in high pressure gas/liquid contact columns can be madelargely independent from the gas pressure by allowing some gas to flowwith the liquid into the downcomers and venting the gas at a lower levelfrom the downcomers. In this way the gas and the liquid flowco-currently in the upper part of the downcomer, thus substantiallyreducing the resistance to the flow of liquid. It will be understoodthat the gas should preferably be removed from the downcomers at a levelas low as possible, closely adjacent to the liquid level duringoperation of such a column.

Referring to FIG. 1, part of a normally vertically extending column 1with cylindrical sidewall 2 is shown. The column is provided with aplurality of substantially horizontal contact trays 3 arranged one abovethe other at suitable intervals, e.g. 150 to 750 mm, and extending oversubstantially the whole cross section of the column. Each of the trays 3is provided with a plurality of apertures 4 forming passages for upwardflowing gas. These apertures 4 are substantially uniformly distributedover the trays and may have any suitable shape, such as slits andcircular holes. The apertures are further of substantially uniform sizethroughout and are spaced apart from each other to provide an aggregatearea, that is the free area of the trays, appropriate to the loadingconditions at which the column is to be operated. The portions of thetrays 3 with the apertures 4 for the passage of gas constitute theactive areas of the trays 3, i.e. above these areas liquid and gasintroduced into the column are intensively mixed with each other for theexchange of matter and/or heat between said two fluid phases.

Adjoining the active tray areas there is on each tray 3 a plurality ofoblong downcomers 5 for descending liquid. The downcomers pass throughthe trays to a predetermined height above the upper surfaces of saidtrays and a predetermined distance below their lower surfaces. The partof a downcomer extending above the tray is normally indicated with theexpression weir. The downcomers are open at their upper ends, whiletheir lower ends are formed by bottomwalls 6 provided with a pluralityof liquid discharge openings 7. The liquid discharge openings 7 aredistributed over the bottomwalls 6 of the downcomers 5 so as todischarge liquid substantially uniformly over the length of saiddowncomers. The aggregate area of the liquid discharge openings 7 ineach downcomer 5 should be sufficient for discharging all of the liquidflowing downwards through the column interior at the intended liquidloading and should be restricted with respect to the horizontal crosssectional areas of the lower parts of the downcomers so as to maintainduring operation of the column in said downcomers a column of liquidexerting at the liquid discharge openings 7 a hydrostatic head which issufficient for preventing ascending gas from entering into thedowncomers via said openings 7. The downcomers are further provided withsplashing baffles to prevent the flow of liquid from one side of adowncomer to the active tray area at the opposite side of the downcomer.These splashing baffles consist of two substantially vertical, parallelwalls 8 arranged at some distance from one another to form a passage 9.The upper ends of the walls 8 should preferably extend above the frothof liquid and gas formed above the trays upon operation of the column atmaximum capacity. It is advisable to have the upper ends of the walls 8arranged closely to the lower surface of the next upper tray 3 above thefroth generated above the trays at maximum loading of the column. Thedistance between the walls 8 should be chosen such that the velocity ofupward flowing gas is relatively small so that liquid entrained with thegas is separated from the ascending gas by gravity flow. Each pair ofparallel walls is preferably substantially coaxially arranged withrespect to the downcomer in which the walls are located. The parallelwalls may be connected to the downcomer walls by any suitable means suchas supporting strips (not shown). The parallel walls 8 are furtherprovided with a plurality of vertically spaced apart slit shapedopenings 10 for the passage of gas. To prevent entry of liquid intothese openings 10, downwardly inclined baffles 11 are secured to thewalls adjacent to said openings.

During operation of the column partly shown in FIG. 1, gas and liquidare caused to flow in countercurrent direction, in such a manner thatgas flows in upward direction via the openings 4 in the trays 3, whileliquid descends through the column via the downcomers 5. At lowpressures the liquid from the froth formed above the trays willsubstantially not be hindered by gas upon entry into the downcomers 5.If the column is operated at elevated pressure the density differencebetween gas and liquid decreases, so that the drag forces on the liquiddroplets become more important. As a consequence of the reduction interminal falling velocity of the liquid droplets, the liquid-handlingcapacity of the downcomer is decreased if no specific measures asproposed in the present application are taken. As already discussed inthe above, the increased resistance to liquid flow may lead to prematureflooding of the column. Furthermore, even at lower than maximum loadingthe liquid droplets will form a barrier resisting the upward flow ofentrained gas. A consequence thereof would be the passage of gas withliquid through the downcomer openings, which phenomenon is usuallyindicated with the expression vapor carry-under.

On account of the arrangement of the parallel walls 8 in the downcomers5 the above problem of vapor carry-under occurring at high gas loads isovercome. Due to the presence of the gas passages formed by the openings10 and the space between the parallel walls gas present in thedowncomers and blocked by the liquid droplets entering into thedowncomers will flow towards the walls and will escape from thedowncomers via said gas passages towards a location above the froth onthe trays. This gas flow will have the effect of reducing the resistanceto flow of the liquid droplets and thus helping the liquid to enter intothe downcomers. The liquid droplet terminal fall velocity isartificially increased due to the forced downward gas flow so that thesuperficial downcomer liquid velocity can increase also without risk ofvapor carry-under.

Reference is now made to FIG. 3 showing a perspective view of part of acolumn tray with alternative gas disengagement means. Because of thedistance the gas has to travel to the disengagement means, anarrangement such as shown in FIGS. 1 and 2, would normally be preferredover the second shown embodiment.

In FIG. 3, the column tray indicated with reference numeral 30 isprovided with apertures 31 for ascending gas and a plurality of oblongdowncomers 32, of which one is shown, with bottom openings 33 fordescending liquid. The downcomers 32 are provided with splashing baffles34 known as such, to cause a pre-separation of gas from liquid prior toentry of the liquid into the downcomers. In this embodiment of theinvention the gas passages for the escape of gas from the downcomersform an integral part of the splashing baffle supporting means. Thesesupporting means consist of pairwise arranged walls 35 secured to thesplashing baffles 34 and the sidewalls of the downcomers 32. The walls35 are provided with openings 36 substantially uniformly distributedover the height of said walls. For protecting the openings 36 againstinflow of liquid, inclined baffles 37 are arranged blanking off saidopenings. The walls 35 are pairwise interconnected via closed end walls38 secured to the downcomer sidewalls. The spaces enclosed by each pairof walls 35, end wall 38 and splashing baffle 34 serve as gas passagesenabling the disengagement of gas from the downcomers. Thereto the upperends of splashing baffles 34, walls 35 and 38 extend at least above thefroth of liquid and gas generated during operation.

For operation of this type of gas passages shown in FIG. 3, reference ismade to the description given with respect to the embodiment of theinvention shown in FIGS. 1 and 2.

The above examples of gas passages with lateral openings in the wallsdefining the gas passages is advantageous for operating at relativelylarge liquid load fluctuations wherein the liquid level in thedowncomers can fluctuate largely. If a column for high pressureoperations is intended to be used at a rather constant liquid load theseside openings in the walls defining the gas passages may be even deletedprovided that the gas entries defined by the lower ends of said wallsare arranged at a level close to but at least above the level of thesettled liquid in the bottom parts of the downcomers.

What is claimed is:
 1. Gas/liquid contacting apparatus comprising:anormally vertically extending column having a plurality of substantiallyhorizontal trays arranged one above the other in the column; each ofsaid trays having a plurality of apertures for ascending gas and aplurality of liquid discharge devices for descending liquid; said liquiddischarge devices being open at their upper ends and extending at leastpartly below the relevant trays with the parts of the liquid dischargedevices extending below the trays being provided with bottom walls whichare pierced by bottom located liquid discharge openings; each of saidliquid discharge devices being also provided with a gas passage which(a) has at least one gas entry opening located within the lower part ofthe liquid discharge device, (b) has walls separated by distances suchthat the velocity of the upward flowing gas is small enough to permitliquid entrained in that gas to be separated from the gas by gravityflow and (c) has an outlet located substantially above the upper end ofthe liquid discharge device; and the areas of said bottom located liquiddischarge openings in said liquid discharge devices being arranged forproviding an aggregate area sufficient for discharging all the liquidflowing downward through the column while maintaining within each liquiddischarge device a column of liquid having a hydrostatic head sufficientto prevent said ascending gas from entering the liquid discharge devicesthrough said bottom located liquid discharge openings, so that thedischarging of liquid through the bottom located openings and theseparating of liquid from gas within the gas passages maintains asubstantially continuous and unhindered entry of liquid into the upperends of the liquid discharge devices.
 2. Gas/liquid contacting apparatusaccording to claim 1, wherein the gas passages are formed byhorizontally spaced splashing baffles extending over substantially thelength of the discharge devices.
 3. Gas/liquid contacting apparatusaccording to claim 1, wherein the discharge devices are provided withsubstantially centrally arranged splashing baffles and the gas passagesare formed by horizontally spaced supporting baffles extending betweenthe splashing baffles and the discharge devices.
 4. Gas/liquidcontacting apparatus according to claim 1, wherein the gas passages arefurther provided with a plurality of vertically spaced lateral gasentries extending over at least the lower part of said gas passages. 5.Gas/liquid contacting apparatus according to claim 4, wherein the upperends of the lateral gas entries are bounded by downwardly inclined wallsfor preventing the entry of liquid.
 6. Gas/liquid contacting apparatusaccording to claim 1, wherein the upper ends of the gas passages of atray are arranged adjacent the next upper tray.
 7. Gas/liquid contactingapparatus according to claim 1, wherein the lower ends of the gaspassages are arranged closely above the bottom walls of the dischargedevices.